Generally, semiconductor devices are highly susceptible to heat, and should be designed such that their internal temperature will not exceed an allowable maximum temperature at a junction area of said semiconductor devices. Semiconductor devices such as power transistors and semiconductor rectifiers consume a large amount of electric energy per operating area, and cases (packages) and leads of such semiconductor devices may not be effective enough to sufficiently radiate heat generated thereby. Therefore, the internal temperature of those semiconductor devices tend to increase to such a level that the semiconductor devices may suffer a thermal breakdown.
The above phenomenon also occurs in semiconductor devices incorporating CPUs. As the clock frequency of the CPU increases, the semiconductor device generates more heat while in operation. It is important to design semiconductor devices to incorporate a thermally designed heat radiation structure.
Thermal designs for protecting semiconductor devices against a thermal breakdown include device designs and mount designs which include heat sinks having a large heat radiating area and fixed to semiconductor device cases (packages).
The heat sinks are generally made of metal such as copper, aluminum, or the like that has good thermal conductivity.
Recently, semiconductor devices such as CPUs, memories, etc. tend to be larger in size because of higher integration of semiconductors and greater areas taken up by semiconductors, while at the same time seeking a low power drive mode for low power consumption. Semiconductor devices of greater size are liable to peel off position or be mechanically broken due to increased stresses which are caused by the difference between the coefficient of thermal expansion of the semiconductor substrate (including a semiconductor device of silicon or GaAs and an insulating substrate of AlN or Si3N4) and the coefficient of thermal expansion of the heat sink.
Possible approaches to the prevention of the above drawbacks include a low power drive mode for semiconductor devices and an improvement of heat sink materials. At present, a practical low power drive mode for semiconductor devices has a power supply voltage of 3.3 V or lower rather than the conventional TTL level (5 V).
As for heat sink materials, thermal conductivity is not the only factor to be taken into consideration for their selection, but it has become necessary to select heat sink materials which have a coefficient of thermal expansion that is substantially the same as the coefficient of thermal expansion of silicon and GaAs which the semiconductor substrate is made of, and also have high thermal conductivity.
Various reports have been made with respect to improved heat sink materials. For example, proposed examples of heat sink materials include aluminum nitride (AlN) and Cu (copper)—W (tungsten). Cu—W is a composite material having a low coefficient of thermal expansion provided by W and a high coefficient of thermal conductivity provided by Cu.
Other examples of heat sink materials include a ceramic base material mainly composed of SiC and containing 20 vol. % to 40 vol. % of Cu (conventional example 1: Japanese laid-open patent publication No. 8-279569) and a powder-sintered porous body of an inorganic material infiltrated with 5 wt. % to 30 wt. % of Cu (conventional example 2: Japanese laid-open patent publication No. 59-228742). However, these heat sink materials hardly satisfy market demands as their characteristics, machinability, and prices are not well balanced.
A conventional electronic component 100 which incorporates a thermal countermeasure therein will be described below with reference to FIG. 15. The electronic component 100 includes an IC chip 108 mounted on a heat sink 102 with a thermally conductive layer 104 and a base layer 106 interposed therebetween. The thermally conductive layer 104 has a laminated assembly 118 mounted on and joined to an Ni plated layer 110 formed by covering the heat sink 102 over. The laminated assembly 118 comprises a lower electrode layer 112 of Cu or Al, an insulating layer (AlN layer) 114, and an upper electrode layer 116 of Cu or Al. The Ni plated layer 110 and the laminated assembly 118 are joined to each other by a solder layer 120. An Ni layer 122 is interposed between the laminated assembly 118 and the solder layer 120 to increase the wettability of the lower electrode layer 112 with respect to the solder layer 120.
The IC chip 108 is mounted on the laminated assembly 118 with a solder layer 124 interposed therebetween. An Ni layer 126 is interposed between the laminated assembly 118 and the solder layer 124 to increase the wettability of the upper electrode layer 116 with respect to the solder layer 124. Similarly, an Ni layer 128 is interposed between the IC chip 108 and the solder layer 124 to increase the wettability of the IC chip 108 with respect to the solder layer 124.
As shown in FIG. 16, another conventional electronic component 200 (see Japanese laid-open patent publication No. 11-307696, for example) has a metal base plate 202 for radiating heat generated by a semiconductor chip 204, a ceramic plate 206 which insulates the semiconductor chip 204 from the metal base plate 202, an upper electrode 210 disposed on an upper surface of the ceramic plate 206 with a brazing layer 208 interposed therebetween, a lower electrode 214 disposed underneath a lower surface of the ceramic plate 206 with a brazing layer 212 interposed therebetween, a metal spacer 216 which spaces the metal base plate 202 and the ceramic plate 206 from each other, a brazing layer 218 which is fixed the metal spacer 216 to the metal base plate 202, a solder layer 220 which is fixed the semiconductor chip 204 to the upper electrode 210, and a solder layer 222 which is fixed the lower electrode 214 to the metal spacer 216.
The electronic component 100 shown in FIG. 15 is manufactured by a process comprising at least eight steps, i.e., the first step of forming the Ni plated layer 110 on the heat sink 102, the second step of forming the Ni layer 128 on the lower surface of the IC chip 108, the third and fourth steps of forming the upper electrode layer 116 and the lower electrode layer 112, each made of Cu or Al, respectively on the opposite surfaces of the insulating layer 114, producing the laminated assembly 118, the fifth and sixth steps of forming the Ni layers 126, 112 respectively on the upper surface of the upper electrode layer 116 and the lower surface of the lower electrode layer 112, the seventh step of joining the laminated assembly 118 to the Ni plated layer 110 on the heat sink 102 with the solder layer 120, and the eighth step of joining the Ni plated layer 128 formed on the lower surface of the IC chip 108 to the laminated assembly 118 with the solder layer 124. The manufacturing process is complex, and leads to an increase in the cost of the final product, i.e., the electronic component 100.
Furthermore, since there are many elements to be laminated to complete the electronic component 100, an attempt may be proposed to reduce the thickness of the solder layer 120 to several hundreds μm, for example, in order to reduce the size of the electronic component 100. However, since the solder layer 120 itself is poor in its heat radiation capability and the electronic component 100 contains many junction interfaces between different materials which tend to impair the heat radiation capability of the electronic component 100, the heat from the IC chip 108 cannot efficiently be transferred to the heat sink 102.
The junction provided by the solder layer 120 is liable to make the electronic component 100 less durable when the electronic component 100 is subjected to heat cycles or heat shocks. Specifically, when the electronic component 100 is subjected to heat cycles or heat shocks, (1) the insulating layer is warped, (2) the electrodes are peeled off, (3) the insulating layer cracks, and (4) the soldered areas suffer cracking, resulting in an operation failure of semiconductor devices contained in the electronic components. The above problems also occur with the electronic component 200 shown in FIG. 16.
The present invention has been made in view of the above problems. It is an object of the present invention to provide a member for use in an electronic circuit which can be manufactured by a process having a greatly reduced number of steps, has its manufacturing cost effectively reduced, and is highly thermally reliable, a method of manufacturing such a member, and an electronic component.